When functioning properly, the human heart maintains its own intrinsic rhythm. Its sinoatrial node generates an intrinsic electrical cardiac signal that depolarizes the atria. Such atrial depolarizations cause atrial heart contractions. Its atrioventricular node then passes the intrinsic cardiac signal to depolarize the ventricles. Such ventricular depolarizations cause ventricular heart contractions. These intrinsic cardiac signals can be sensed on a surface electrocardiogram (e.g., a “surface ECG signal”) obtained from electrodes placed on the patient's skin, or from electrodes implanted within the patient's body (e.g., an “electrogram signal”). The surface ECG and electrogram waveforms generally include artifacts associated with atrial depolarizations (“P-waves”) and those associated with ventricular depolarizations (“QRS complexes”).
When people have irregular cardiac rhythms, referred to as cardiac arrhythmias, or poor spatial coordination of heart contractions, diminished blood circulation may result. For such persons, a cardiac rhythm management (CRM) system may be used to improve these conditions. CRM systems include, among other things, pacers which deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart. By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, improving blood pumping efficiency. Another type of CRM system includes defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators include cardioverters, which synchronize the delivery of such stimuli to sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, which include abnormal heart rhythms that are typically characterized by a rapid heart rate. Tachyarrhythmias generally include supraventricular tachyarrhythmia (SVT), such as atrial tachyarrhythmia (AT), and ventricular tachyarrhythmia (VT). Tachyarrhythmia can also include fibrillation, such as ventricular fibrillation (VF), which is characterized by an irregular heart rhythm.
In a normal heart, the sinoatrial node, the heart's predominant natural pacemaker, generates electrical impulses, called action potentials. The action potentials propagate through an electrical conduction system to the atria and then to the ventricles of the heart to excite the myocardial tissues. The atria and ventricles normally contract in atrio-ventricular sequence and synchrony to result in efficient blood-pumping. VT typically occurs when the electrical impulses propagate along a pathologically formed self-sustaining conductive loop within the ventricles, or when a natural pacemaker in a ventricle usurps control of the heart rate from the sinoatrial node. When the contractions of the atria and the ventricles become dissociated during VT, the ventricles may contract before they are properly filled with blood, resulting in diminished blood flow throughout the body. This condition can become life-threatening, such as when the brain is deprived of sufficient oxygen supply.
Cardioversion and defibrillation can be used to terminate most tachyarrhythmias, including AT, VT, and VF. An implantable cardioverter/defibrillator (ICD) is a CRM device that delivers an electric shock to terminate a detected tachyarrhythmia episode by depolarizing the entire myocardium simultaneously and rendering it refractory. Another type of electrical therapy for tachyarrhythmia is anti-tachyarrhythmia pacing (ATP). In ATP, the heart is competitively paced in an effort to interrupt the reentrant loop causing the tachyarrhythmia. An ICD can include ATP and defibrillation capabilities so that ATP is delivered to the heart when a non-fibrillation VT is detected, while a defibrillation shock is delivered when VF occurs.
The efficacy of cardioversion, defibrillation, and ATP in terminating a tachyarrhythmia can depend on the type and origin of the tachyarrhythmia. An unnecessary shock delivered during a non-life-threatening tachyarrhythmia episode may cause substantial pain in the patient, and reduces the longevity of the ICD while providing the patient with little or no benefit. On the other hand, a necessary therapy that is improperly withheld during a life-threatening tachyarrhythmia episode may result in irreversible harm to the patient, including death.